1. Field of the Invention
This invention relates to an organic electroluminescent device (which will hereinafter be called "organic EL device" simply) used for a planar light source or display device, particularly to a red-light emitting organic EL device.
2. Description of the Related Art
The organic EL device is considered promising as a spontaneous emission type planar display device. The organic EL device is expected to find its utility for a full color display and is therefore under brisk development, because different from an inorganic EL device which requires AC drive and high voltage, it is free from such limitations and in addition, is presumed to facilitate multi-color emission owing to a variety of usable organic compounds. When the above-described organic EL device is applied to a full color display, it is necessary for the device to emit three primary colors, that is, red, green and blue.
A number of reports have been published on green light emission and reported as a green light emitter are, for example, a device using tris(8-quinolinol)aluminum [Applied Physics Letters, 51, 913(1987)] and a device using a diarylamine derivative (Japanese Patent Application Laid-Open No.53397/1996).
There are also many reports on a blue-color emitting device, for example, a device using a stilbene compound (Japanese Patent Application Laid-Open No. 295359/1993), a device using a triarylamine derivative (Japanese Patent Application Laid-Open No. 53955/1995), a device using a tetraaryldiamine derivative (Japanese Patent Application Laid-Open No. 48656/1996) and a device using a styrylated biphenyl compound (Japanese Patent Application Laid-Open No. 132080/1994). In addition, it has been reported that a distyrylarylene derivative used as a light emitting material exhibited brightness of at least 20000 cd/m.sup.2, luminous efficiency of 5 lm/W and half life of at least 5000 hours (special lecture at the 70th Spring Annual Meeting of The Chemical Society of Japan).
Concerning an organic EL device from which red light emission is available, red light emission is obtained by the conversion of the wavelength of blue light emission in a fluorescent dye layer according to Japanese Patent Application Laid-Open No. 152897/1991, while it is obtained by doping of a red fluorescent dye into an emitter layer from which green or blue light emission is available according to Japanese Patent Application Laid-Open Nos 272854/1995, 288184/1995 or 286033/1996. Neither is sufficient from the viewpoints of brightness and color purity. Japanese Patent Application Laid-Open No. 791/1991 discloses an organic EL device using a red fluorescent dye singly for an emitter layer. Although Japanese Patent Application Laid-Open No.93257/1994 discloses the use of a squarylium compound represented by the following structural formula: ##STR2##
as a dopant in an emitter layer, red light emission of the resulting device is not sufficient in color purity. Further improvement is necessary for the practical use of it.
The above-described method according to the conversion of the wavelength of blue light emission in a fluorescent dye layer is also accompanied with the problems that sufficient luminous efficiency cannot be attained because a quantum yield for color conversion of EL light emission through the fluorescent dye layer is limited; and the use of the layer inevitably raises the production cost.
In addition, the red-light emitting material exemplified in the above conventional example has a low quantum yield of fluorescence and even by an increase in the electric current flowing inside of the device, light is emitted only at brightness of about 1000 cd/m.sup.2 so that it is not suited for practical use.